激光光内宽带送粉增材制造光粉耦合机制及分光束预热缓冷效应

Wide-band metal laser cladding has the advantage of high efficiency, flat cladding layer, low unbalanced hardness and heat stress coursed by thermal cycles. However, this method also shows the difficulty in laser powder coupling, low powder usage and high void rate. This project innovates a new idea which is to feed the powder inside a hollow wide-band laser, meanwhile, the original laser is also split to another two extra beams at the sides which are for pre-heating the substrate forehead and slow-cooling afterward. This is a new technology with four laser beams and powder feeding from the very middle. Among the four beams, the center of the dual beams in the middle is aligned with the center of the powder stream, and this doesn’t make sure the air-carrying powders is always feeding vertically into the melted pool, but the powder and the dual laser are also accurate coupled either at all scanning directions or at varying focus positions. It is seen that the powder usage is improved, part forming is achieved at large working angle and varied scanning directions, heat stress and defect rate are reduced by pre-heating/slow cooling process. The key research points in this project will be the structural design of the four split laser beams, the coupling principle and effect among the hollow laser, powder and compressed air, the heat treatment principle and effect of the pre-heating and slow-cooling process. These points will provide a new theory and method for inside powder feeding wide-band hollow laser additive manufacturing technology, and improve the quality and the efficiency when laser forming the large metal components.

金属宽带激光熔覆效率高、熔道平整、可减少窄带熔覆反复加热带来的组织硬度不均衡和热应力等，但目前尚存在光粉耦合精度较低、粉末利用率低、缺陷率较高等问题。本项目提出双聚焦激光束光内宽带送粉熔覆、双低密度光束前置预热和后置缓冷的新方法，形成4光束中心送粉宽带熔覆增材制造新技术。聚焦双线型熔覆光斑中心始终与宽带粉束中心重合，准直气包夹宽带粉束垂直熔池喷射，往复扫描和离焦波动条件下均能保证光粉精确耦合；粉末利用率大大提高；空间大倾角和变方向动态熔覆成形的运动熔池稳定；预热缓冷光斑辐照可降低熔层残余应力和缺陷几率。重点研究中空宽带4光束调制变换，宽带双聚焦光束—光内送粉—准直气在空间与熔池相互耦合的新机理与新效应、预热与缓冷光束对熔池前后方随动辐照的热处理作用机制与效应，为形成新的光内宽带送粉成形工艺提供理论与方法基础，促进大型金属零构件的高质高效激光增材制造技术进步。

金属宽带激光熔覆效率高、熔道平整、可减少窄带熔覆反复加热带来的组织硬度不均衡和热应力等，在大型工件表面的强化、修复和改性中的应用很有优势。针对现有激光宽带熔覆存在光粉耦合精度较低、粉末利用率低、缺陷率较高等问题，本项目提出双聚焦激光束光内宽带送粉熔覆、双低密度光束前置预热和后置缓冷的新方法，形成宽带光束中心送粉宽带熔覆增材制造技术。熔覆光斑中心始终与宽带粉束中心重合，准直气包夹宽带粉束垂直熔池喷射，往复扫描和离焦波动条件下均能保证光粉精确耦合,粉末利用率大大提高,空间大倾角熔覆成形的运动熔池稳定。预热缓冷光斑辐照可降低熔层残余应力和缺陷几率。本项目提出一种可以获得大宽度匀化光斑的抛物带式矩形光斑积分镜，实现了在宽带激光束内部均匀、准确地垂直于熔池表面送粉。应用TracePro光学软件选取较优偏转角(θ=60°)进行镜面设计，得到了光通量均匀分布的双线形聚焦光斑。.在大匀化宽度积分镜设计的基础上，设计开发出20kW宽带光内送粉激光熔覆光学系统。通过90°直角锥镜分光，再反射会聚实现高功率宽带光内送粉激光熔覆光学系统，解决了高功率激光光学的设计、加工、镀膜、光场匀化、分光汇聚、光粉耦合多个技术难题。.采用研制的宽带光斑光内送粉激光宽带熔覆喷头进行单道熔覆实验，获得了表面较为光洁、无裂纹、有金属光泽的熔覆层，熔覆粉末的利用率约为60%。通过光内送粉多道搭接熔覆实验，获得了表面较为平整、光洁的熔覆层。经熔覆层组织分析、显微硬度的检测可知，搭接区组织较非搭接区粗大、硬度低，非搭接区组织主要由较小的树枝晶组成，熔覆层组织整体均匀。本项目研究实现了新型宽带激光束内部送粉的熔覆工艺，并可用于小角度斜壁表面熔覆和堆积成形，为空间立体成形和修复提供了一种新的工艺和方法。